Material audit
6063-T5 Aluminium Profile: What the Designation Actually Guarantees
The designation on a drawing looks like a guarantee. It is closer to a route description: it says how the profile was cooled and aged, and nothing about whether the delivered metal meets the numbers you designed to. Here is what each part of the label commits the mill to, where the strength actually comes from, and the one place where welding quietly voids the whole thing.
Quick answer
- In EN 515 and ANSI H35.1, T5 means the profile was cooled from an elevated temperature shaping process and then artificially aged. It was never given a separate solution heat treatment. T6 means it was solution heat treated, quenched and then aged. That single difference explains almost everything else about the two tempers.
- Because T5 relies on the cooling that happens as the metal leaves the die, the cooling rate — and therefore the wall thickness — changes the properties. EN 755-2 gives 6063-T5 a minimum tensile of 175 MPa and yield of 130 MPa for walls up to 3 mm, falling to 160 MPa and 110 MPa for walls from 3 to 25 mm.
- The same temper under ASTM B221-02 is a weaker promise: 152 MPa tensile and 110 MPa yield up to 12.7 mm, and 145 MPa and 103 MPa from 12.7 to 25.4 mm. EN 755-2 asks for roughly 15 per cent more tensile and 18 per cent more yield than ASTM B221 for the identical 6063-T5 designation. A certificate that names one standard is not evidence for the other.
- The modulus of elasticity is 69 to 70 GPa regardless of temper, and the density is 2.70 g/cm³. Neither number is a function of T5, T6 or T66, and neither can be improved by paying for a higher temper.
- Stretching after the quench is what makes the profile straight and relieves the internal stress. It is normally 0.5 to 3 per cent, targeted around 1 per cent, and should not exceed 2 per cent. Above that, the head, middle and tail of the length start to differ in size.
- Welding removes the T5 condition locally. The heat affected zone is partially annealed and over-aged, and published groove-weld data for 6063-T6 shows tensile dropping from 31 to 19 ksi, a retention of about 61 per cent. A welded 6063-T5 frame should be designed on roughly 65 to 80 MPa yield in the joint area, not on the 110 to 130 MPa of the parent metal.
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What each part of the label commits the mill to
Read a material callout as four separate statements, because each one is verified by a different thing.
| Part of the label | What it commits to | How it is verified |
|---|---|---|
| 6063 | Chemical composition: Si 0.20–0.60, Mg 0.45–0.90, Fe 0.35 max, Cu 0.10 max, balance aluminium | Cast analysis on the billet, reported on the mill certificate |
| T5 | Cooled from the extrusion temperature and artificially aged, with no separate solution treatment | Route record from the press and the ageing oven log |
| Minimum properties | Whichever standard is named, and the thickness band that applies | Mechanical test on a sample cut from the same lot |
| Dimensional tolerances | EN 755-9 or the equivalent national standard, at the agreed tolerance class | Measurement of the delivered profile, not of the certificate |
Note what is not in the label. There is no statement about straightness, twist, surface finish, residual stress or the position of the profile within its tolerance band. Those are governed by other clauses, or by nothing at all unless you write them into the order. The designation is a material commitment; it is not a part commitment.
The most common specification error we see. A drawing reads “6063-T5” and nothing else. The supplier quotes against ASTM B221 because that is the standard on their certificate. The designer assumed the European values. Both parties believe they agreed, and the delivered metal satisfies the weaker of the two. Write the standard and the thickness band into the callout: 6063-T5 to EN 755-2, wall thickness 2.0 mm.
The numbers, by standard and by wall thickness
This is the table that decides whether a delivered lot conforms. Both columns describe the same alloy and the same temper. They are not the same requirement.
| Standard | Wall or thickness band | Tensile Rm min | Yield Rp0.2 min | Elongation min |
|---|---|---|---|---|
| EN 755-2, 6063-T5 | up to 3 mm | 175 MPa | 130 MPa | — |
| EN 755-2, 6063-T5 | 3 to 25 mm | 160 MPa | 110 MPa | — |
| ASTM B221-02, 6063-T5 | up to 12.7 mm | 152 MPa | 110 MPa | 8 % |
| ASTM B221-02, 6063-T5 | 12.7 to 25.4 mm | 145 MPa | 103 MPa | 8 % |
| ASTM B221-02, 6063-T6 | up to 25.4 mm | 207 MPa | 172 MPa | 8–10 % |
Two things to notice. First, the ASTM rows are weaker at every band, so a supplier who certifies to ASTM has not demonstrated EN compliance even though the temper symbol is identical. Second, both standards make the properties worse as the wall gets thicker, which is counter-intuitive until you look at the process.
Why a thicker wall is a weaker wall
In a T5 route there is no separate solution treatment. The magnesium and silicon are taken into solution during the extrusion itself, and then locked in place by the cooling that follows the die. That cooling is the only quench the alloy ever gets. A thin wall loses heat quickly and freezes a supersaturated solid solution; a thick wall cools slowly, some of the hardening elements precipitate as coarse particles on the way down, and less of the alloy is available for the fine precipitation that the ageing step is supposed to produce.
The consequence is that the property table is not a safety margin for thick sections. It is a statement that thick sections are metallurgically different. A 6 mm wall in the same lot as a 2 mm wall is not just slower to cool; it is a weaker piece of metal, and the standard says so explicitly.
The ageing step only works on what the quench preserved
T5 artificial ageing is typically carried out in the range of 170 to 200 °C for a few hours. It precipitates the hardening phase from a supersaturated solution. If the quench was too slow and the solute already precipitated coarsely, no ageing cycle recovers it. This is why the same oven, the same cycle and the same alloy can produce conforming thin sections and non-conforming thick ones in the same load.
The route that produces T5, step by step
The temper symbol is an output. These are the steps that produce it, and each one is a place where a delivered profile can miss the drawing without any of the paperwork looking wrong.
Billet heating and the die
The billet is heated to roughly 450 to 500 °C and pushed through the die. The die decides the wall thickness distribution, and the wall distribution decides how evenly the section cools. A 2 per cent difference in wall thickness between two sides of the same profile looks harmless on a strength calculation, but it produces an uneven metal flow that shows up later as bow or twist.
The quench, which is the whole of T5
Cooling starts the moment the metal leaves the die. Thin and simple sections can be air cooled; thicker sections need water or a mist, and a profile that is air cooled when it needed water will fail its tensile test even though it is correctly labelled T5. Uneven spray produces uneven cooling, which produces internal stress and, in the worst case, a bow that appears a day after the profile was measured straight.
Stretching
After cooling, the profile is gripped at both ends and pulled to a controlled permanent set, normally 0.5 to 3 per cent and targeted around 1 per cent. This is not cosmetic. Its purpose is to relieve the longitudinal stresses left by the quench, which are high enough to distort the profile on the runout table. The published guidance is that the amount should not exceed 2 per cent, because beyond that the head, middle and tail of the length start to differ in cross-section, elongation drops and hardness rises.
Cutting to length and ageing
The stretched profile is cut and loaded into the ageing oven. Ageing is what raises the yield strength from the as-quenched condition to the T5 values, and it is a time and temperature pair, not a temperature. A load that sits below the set point for the full cycle will test below the minimum even though the oven chart shows the correct temperature.
The suffix that tells you which route was used. EN 515 adds stress-relief suffixes to the temper. Tx51 means stress relieved by stretching. Tx510 means stress relieved by stretching with no further straightening afterwards. Tx511 means stress relieved by stretching with minor straightening allowed afterwards to meet tolerance. So 6063-T510 and 6063-T511 are not different alloys or different strengths — they are more precise statements about what happened to the profile after the quench. If your drawing specifies plain T5 and the certificate says T510, ask why, because the two routes have different residual stress.
The correction that matters most: welding
Welding is the single largest source of disagreement between what a 6063-T5 drawing assumes and what the finished assembly can actually do. It is also the point where most published material on this alloy is simply wrong.
Fusion welding melts the parent metal and conducts heat into the surrounding section. That heat cycle does to the heat affected zone roughly the opposite of what the ageing oven did. Published guidance describes the HAZ in a welded heat-treatable alloy as partially annealed and over-aged, and states the rule plainly: the higher the heat input, the lower the as-welded strength.
| Alloy and temper | Parent tensile | As-welded tensile | Retention |
|---|---|---|---|
| 6063-T6 | 31 ksi (214 MPa) | 19 ksi (131 MPa) | 61 % |
| 6061-T6 | 45 ksi (310 MPa) | 27 ksi (186 MPa) | 60 % |
| 6061-T4 | 35 ksi (241 MPa) | 27 ksi (186 MPa) | 77 % |
| 7005-T53 | 57 ksi (393 MPa) | 43 ksi (296 MPa) | 75 % |
The pattern is consistent and it is not specific to 6063. A welded joint in an aged 6xxx alloy keeps roughly 60 per cent of its tensile strength in the zone next to the weld. Applying that retention to 6063-T5 gives an HAZ tensile of about 105 MPa and, scaling the yield by the same factor, an HAZ yield of roughly 65 to 80 MPa.
What this means on a drawing. If a frame is welded and the welded joint carries bending, the allowable stress at that joint is not 110 or 130 MPa. It is closer to 65 MPa. Design the joint on that number and the parent metal will not be the problem. Design it on the parent number and the frame will pass every calculation and still fail at the weld.
There is a second, quieter consequence. The T5 condition no longer exists in the HAZ, so a welded frame cannot honestly be described as a 6063-T5 structure. If the temper matters to the customer, the joint should be bolted, or the weld should be followed by a full re-heat-treatment that most fabricators cannot perform on a finished frame.
The practical alternatives
- Bolt instead of weld. A bolted T-slot joint keeps the parent metal in its certified condition everywhere except under the bolt heads. This is the main structural argument for modular framing, and it is stronger than most buyers realise.
- Keep the weld away from the high-moment region. If a welded joint must exist, put it where the bending moment is low. On a beam, that means near the supports, not at mid-span.
- Increase the section at the joint, not the alloy. Since the modulus does not change and the HAZ strength is roughly fixed as a fraction of the parent, a larger section or a gusset helps; a higher temper does not, because the HAZ loses the same proportion of a bigger number.
- Specify 5083 or a 5xxx filler for a welded aluminium frame. Non-heat-treatable alloys do not lose strength through over-ageing in the same way, because their strength never came from precipitation in the first place.
Where 6063-T5 is the wrong answer
6063-T5 is the default extrusion temper for good reasons: it extrudes faster and into more complex shapes than 6061, it takes a better anodised finish, and it costs less. Those advantages come with limits, and knowing them is more useful than a list of benefits.
| Requirement | Why 6063-T5 struggles | What to use instead |
|---|---|---|
| High load on a short span, where yield governs | Yield is 110 to 130 MPa, the lowest of the common structural grades | 6061-T6 at 276 MPa, or a larger 6063 section |
| Welded primary structure | The HAZ drops to roughly 60 per cent of parent strength | Bolted joints, or a 5xxx alloy |
| Hard anodising for wear resistance | The alloy is too soft to build a thick hard anodic layer reliably | 6061-T6, which hard anodises well |
| Heavy machining of the finished part | Soft and gummy, produces long chips and built-up edge | 6061-T6 for chip formation |
| Tight tolerance on a thick wall | Slow quench leaves the properties at the bottom of the band | 6063-T6 or T66, or a thinner wall |
A useful way to hold the two tempers in mind. T5 is the temper you get for free from the extrusion process. T6 and T66 are tempers you buy, because they require a separate solution treatment, a separate quench and the distortion that comes with it. T66 reaches about 245 MPa tensile and 200 MPa yield, which is a real gain — but it is a gain in strength, not in stiffness, and it comes with tighter dimensional control problems. Choose it when yield governs, not when deflection does.
Auditing the mill certificate
A mill certificate is a short document and most buyers read only the alloy line. These are the lines that decide whether the metal matches the drawing.
None of this requires a laboratory. It requires the drawing and the certificate to be read side by side, which takes about two minutes per lot and prevents the most expensive class of dispute in this trade: metal that conforms to a standard nobody agreed to.
Frequently asked questions
What does T5 actually mean in 6063-T5?
In the temper designation systems used by EN 515 and ANSI H35.1, T5 means the product was cooled from an elevated temperature shaping process and then artificially aged. For an extrusion, the shaping process is the press, so the cooling that follows the die is the only quench the alloy receives, and the ageing oven does the rest. T6 means something different: solution heat treated, then quenched, then artificially aged. Because T6 includes a separate solution treatment, it produces higher and more uniform properties than T5, and it costs more because of the extra thermal cycle and the distortion that comes with it.
Is 6063-T5 weldable?
It welds easily — it is routinely joined by TIG and MIG with 4043 or 5356 filler — but welding removes the T5 condition in the heat affected zone. The HAZ is partially annealed and over-aged by the heat input, and published groove-weld data for 6063-T6 shows tensile strength dropping from 31 to 19 ksi, a retention of about 61 per cent. Applied to 6063-T5 that puts the HAZ yield at roughly 65 to 80 MPa against 110 to 130 MPa in the parent metal. The joint is weldable; it is just not T5 any more, and the design must account for that.
Why does a thicker wall have lower certified properties?
Because in a T5 route the cooling rate after the die is the only quench. A thin wall cools fast enough to hold the magnesium and silicon in supersaturated solution, which is what the ageing step then precipitates into fine hardening particles. A thick wall cools slowly, so some of those elements precipitate coarsely on the way down and are no longer available to contribute strength. The standards reflect this by giving lower minimums above a thickness threshold: EN 755-2 steps from 175 to 160 MPa tensile and from 130 to 110 MPa yield when the wall passes 3 mm.
Is 6063-T5 or 6063-T6 better for a machine frame?
It depends on which limit governs, and for most machine frames that is deflection rather than yield. Both tempers have the same modulus of elasticity, about 69 to 70 GPa, so they deflect identically at the same load. T6 only helps where the frame is yield-limited, which for a 40 x 40 section simply supported is below roughly 1,020 mm of span. Above that the frame is deflection-limited and T6 buys nothing measurable. Use T5 for the frame and spend the difference on a larger section or an extra support.
Can I anodise 6063-T5?
Yes, and it is the best of the common extrusion alloys for it. The low iron content, 0.35 per cent maximum, and the near-absence of copper keep the anodic layer clear and uniform, which is why architectural anodised profiles are almost always 6063. What T5 does not give you is hardness. The alloy is relatively soft, so it takes a decorative or protective anodic layer well but is not the right choice for thick hard-anodised wear surfaces. For those, 6061-T6 responds better to hard anodising despite its worse appearance in decorative finishes.
How do I specify 6063-T5 so the supplier cannot substitute a weaker standard?
Write four things into the material callout instead of two: the alloy, the full temper designation including any stress-relief suffix, the standard with its revision year, and the wall thickness band. For example: 6063-T5 to EN 755-2, wall 2.0 mm, tolerances to EN 755-9:2016. Then ask for the mill certificate to name the same standard and to report actual test values rather than restating the minimums. If a quotation comes back against ASTM B221 instead, the properties are 152 MPa tensile and 110 MPa yield rather than 175 and 130 for a thin wall, and you should decide whether that is acceptable before the metal is cut.
Send us the drawing and the wall thickness
We will confirm the temper, the standard and the certified minimums that apply to your section, and tell you plainly if the joint detail is the part that needs changing.
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